The present invention relates to microelectronic assemblies and to methods for manufacturing such assemblies. Some embodiments provide reliable assemblies and simplify some aspects of the manufacturing process.
FIG. 1 illustrates a typical microelectronic assembly in which a semiconductor die 110 is attached to a wiring substrate (WS, e.g. printed circuit board) 120. The die was manufactured as part of a semiconductor wafer in a single process with other die, and was then separated from the wafer. The die's contact pads 110C are attached to contact pads 120C of WS 120. The attachments 124 can be solder for example. WS 120 has conductive lines 120L used to connect the die to other circuitry (not shown).
A common source of failure in microelectronic assemblies is cracking or detachment of connections 124 due to thermal stresses, i.e. mechanical stresses caused by thermal expansion and contraction that may occur during manufacture and subsequent use (operation) of the assembly. In particular, wiring substrates 120 are commonly made using organic or ceramic materials that have higher coefficients of thermal expansion (CTE) than the die's semiconductor materials (such as silicon). Therefore, when the assembly is heated, the WS expands more than the die, pulling the connections 124 sideways and causing damage. Further, such thermal stresses may warp the assembly and make it harder to connect the assembly to other circuits. In order to reduce the stresses on connections 124, underfill 130U (an adhesive) is introduced between the die and substrate 120 to glue the die to the substrate. However, a typical underfill is based on organic polymers which themselves have high CTEs and thus tend to increase the warpage.
To lessen the warpage, the assembly can be strengthened by a lid 210 (FIG. 2) glued to die 110 and WS 120 by adhesive generally shown at 130A. The lid can be made of metal because metals have high CTEs similar to the CTEs of organic polymers used in WS 120 and in adhesive 130A and 130U. In addition to strengthening the assembly, the metal serves as a heat sink removing heat from the assembly during manufacture and operation.
The assembly reliability may depend on the fabrication sequence; see U.S. pre-grant patent publication no. 2014/0091461 (Apr. 3, 2014; inventor: Shen). In a typical fabrication sequence, the die is attached to the WS, then underfill 130U is introduced in liquid form between the die and the WS. At this point, since the underfill is liquid (and therefore has a low elastic modulus), the underfill does not exert significant warping pressure on the WS. Then the underfill is cured to solid state (by heat for example); the underfill shrinks and may warp the structure. Then lid 210 is attached by adhesive 130A. The lid has to be pressed hard against the warped assembly to straighten the assembly. This pressure may break the assembly, and even if the assembly is not broken the assembly remains stressed, and this residual stress may break the assembly later on, especially when additional stresses are introduced by subsequent fabrication steps or during operation.
An alternative process suggested by the aforementioned patent publication 2014/0091461 is to attach the lid when the underfill is still liquid (uncured) or even before the underfill is deposited. For example, in FIG. 3A, die 110 was attached to the substrate, then lid 210 was attached to the die (by adhesive 130A) before the underfill 130U deposition. The structure was not affected by the underfill during the lid attachment, so the warpage was low or absent. Moreover, the lid is not directly attached to substrate 120, so the substrate warpage is less important for the lid attachment. Therefore, less pressure is needed for the lid attachment. Then (FIG. 3B) underfill 130U is introduced in liquid form and cured to directly attach both the die and the lid to the WS. During the curing process, the lid constrains the die's thermal deformation, and this may further reduce warpage caused by the curing process.
Other improvements are desired, especially if the die 110 is replaced by a multichip module (MCM) possibly containing a stack of die.